Linear low-density polyethylene (LLDPE) possesses properties that distinguish it from other polyethylene polymers. Examples of such properties are described in U.S. Pat. No. 4,076,698 by Anderson et al. The use of LLDPE is growing rapidly in markets such as blown and cast films, injection molding, rotational molding, blow molding, pipe, tubing, and wire and cable applications. The principal use for LLDPE copolymers is in film forming applications because the copolymers typically exhibit high dart impact, high Elmendorf tear, high tensile strength and high elongation, in both the machine direction (MD) and the transverse direction (TD).
Ziegler-Natta type catalyst systems for the producing polyethylene and LLDPE are well known in the art and have been known at least since the issuance of U.S. Pat. No. 3,113,115, by Karl Ziegler et al. U.S. Pat. No. 3,787,384, by Steven et al., and U.S. Pat. No. 4,148,754, by Strobel et al., describes a catalyst prepared by reacting a support (e.g., silica containing reactive hydroxy groups) with an organomagnesium compound (e.g., a Grignard reagent) and then combining this reacted support with titanium compounds. French Patent No. 2,116698, by Durand et al., describes a catalyst prepared by a method comprising a reaction between magnesium metal, at least one halogenated hydrocarbon, and at least one tetravalent titanium compound. European Patent EP 0,529,977, by Eric Daire, describes using such a catalyst composition in a gaseous phase process to produce LLDPE. The catalyst composition of Durand et al. and the corresponding gaseous phase polymerization process of Daire do not produce LLDPE with a density of less than 0.918 at high catalyst productivity rates because of poor fluidization caused by resin stickiness, chunk formation, and reactor fouling. It would be advantageous and desirable to devise a catalyst system to produce LLDPE resins of lower density with reduced resin stickiness, chunk formation and reactor fouling in the fluid bed gas-phase process, especially at high production rates.
Recently, additional efforts to improve the polymer physical and chemical properties so as to be useful in a wide variety of superior products and applications have focused on (1) increasing catalyst productivity to reduce LLDPE cost; (2) narrowing molecular weight distributions of the resins produced with the catalysts; (3) increasing the capability of the catalysts to effectively co-polymerize ethylene and alpha-olefins; (4) reducing content of lower molecular weight component; and (5) improving the response of the resin molecular weight to hydrogen.
Specifically, copolymers of ethylene with α-olefin (e.g., LLDPE) having narrower molecular weight distribution are desired for films and injection-molded products. Resins having a relatively narrow molecular weight distribution produce inject-molded products exhibiting a minimum amount of warping or shrinkage. Additionally, LLDPE resins having relatively low molecular weight distribution produce films having better strength properties than resins with broad molecular weight distribution.
Catalysts with the ability to effectively co-polymerize ethylene with higher alpha-olefins such as C3-C10 alpha-olefins to produce polymers having low densities are desired. Enhancing the incorporation of co-monomer into the polyethylene chain reduces polymer density, giving the LLDPE advantageous properties. LLDPE with lower density is used to produce polyethylene film that is substantially more resistant to tearing and puncturing, has higher dart impact, and better cling properties than a film made from similar resins of higher densities.
It is also desirable to provide a catalyst composition with good hydrogen response, capable of producing ethylene copolymer products in the gas phase polymerization.